Reverse transcriptase (RT) and p30 from several retrovirus have been shown to bind together specifically to form a high molecular weight complex. This complex displays increased DNA-polymerase activity over that shown by RT alone. Evidence suggests that the spontaneous appearance of an endogenous p30-RT complex in thymocytes may be important for the subsequent neoplastic transformation of the AKR thymus. The derivation and nature of this complex is under investigation.